I wouldn't be surprised if Apple bended that 4 year rule, though. But also realize that Apple will eventually stop repairing all computers just due to time passage. And also, eventually, from a financial perspective it's more cost effective to find a working used computer (after heavy depreciation) than to actually do a repair even if that repair is doable.
I do think that their replacement keyboards have better reliability than the early models. I didn't have any problems after getting mine replaced - I believe I used the replacement keyboard for around two years. But I still wasn't willing to keep the computer longer. I went straight from the 2016 to the 2019 16-inch MacBook Pro.
In any event, it was a nice opportunity for an upgrade. My 2016 was still worth $1000, and I ended up with 4 more cores, double the storage, and an absolutely massive increase in graphics performance. Using the education store and picking up in a state with no sales tax did a lot to make that price more palatable.
Unfortunately, Apple will only replace the butterfly keyboard for 4 years after initial retail purchase of the laptop.
I dread the eventual breakdown of the new butterfly keyboard after the end of February next year, when I'm on my own. I hope iFixIt will start selling DIY repair kits, or the cost to repair at Apple makes such a frequent failure a non-starter (in which case I'll turn it into a fixed-place computer in the house, with an external keyboard).